Historically, motor operated valves have been employed in applications where factors such as line size, high pressure, temperature, flow rate, or inaccessible location dictates that other types of valve operators will not suffice. In nuclear power facilities, the additional concerns of radiological exposures, fast emergency system operation, and the required ability to bring the plant to a safe shutdown condition underscore the critical role of properly functioning motor operated valves (MOVs).
In recent years, the issue of MOV reliability has become the subject of increasing concern, particularly in the nuclear power industry. Hundreds of MOV failures have been investigated in different studies.
In one study, Assessment of Motor Operated Valve Failures, INPO-83-037, Institute of Nuclear Power Operations, Atlanta, Ga. (October, 1983), electromechanical torque switches and limit switches were identified as the components at the root of approximately 32 percent of the documented MOV failures. Mechanical failures (failure to operate, bent stems, damage to valve seats, gear binding and damage) accounted for 22 percent of the MOV failures analyzed in the INPO report. Thus 54 percent of the MOV failures were attributable to electromechanical components within the actuator itself.
As a result, it is clear that the electromechanical components within the motor actuator are in need of improvements. A related problem is the difficulty in accurately setting up the torque and limit switch set points. In particular, prior art torque switches typically have set screws for setting the torque limit points without any accurate mechanism for correlating the set screw position to a specified amount of valve stem load.
In addition to their unreliability, a problem with limit switches on prior art valve motor operators (VMOs) is that they normally provide no information as to to position of the valve stem in mid-stroke. Thus it is impossible to ascertain, for example, if the set point is going to be attained momentarily, or if a set point is being approached more rapidly than normal (e.g., because of an overvoltage condition) or more slowly than normal (e.g., because of increased stem packing friction or gear train binding).
Another shortcoming of prior art valve motor operators is the accuracy of thermal overload protection that is typically provided in VMOs. A snap action bimetallic switch is exposed to an electrical resistance heater through which motor winding current flows. If the motor current increases inordinately, and for a sufficient period of time, the heat from the resistance heater causes the bimetallic switch to open, thereby opening the motor starter coil and shutting down the motor. This method of detecting motor overload is indirect and unreliable. A direct measurement of motor load is much preferable because this would be more consistent with typical motor specifications which include limitations on the length of time that the motor can withstand different overcurrent conditions.
The present invention overcomes many of the shortcomings of prior art valve motor operators. Limit switches are replaced by measurements of actual valve stem position. Torque limit switches are replaced by measurements of valve stem load. Snap action bimetallic switches for motor thermal overload protection are replaced with measurements of actual motor load. All of these measurements are periodically compared by a microprocessor with corresponding set points at least fifty times per second.
A significant feature of the present invention is that it can use successive valve stem position measurements and valve load measurements to predict the valve stem position and valve stem load at a specified time in the future. Using this capability, the control system of the present invention takes the inertia of the system into account by turning off the motor when the predicted future value of either the valve stem position and/or the valve load at a specified time in the future (e.g., 0.20 seconds in the future) exceeds the corresponding set point. Thus motor cutoff can be initiated before the valve reaches or exceeds a set point. This has been found to greatly reduce overshoot problems caused by rotational inertia. When opening or closing a valve, overshoot puts unnecessary stress on the valve and can cause valve failure; when positioning a valve at an intermediate position for flow control, overshoot causes the valve motor to be turned on and off (or reversed) more than necessary, which may be harmful to the motor.
Another benefit of the present invention is that tailoring of the control system to various applications is much simpler than with prior art systems because most adjustments require only the change of parameter values in the control software, or modification of the software, rather than the redesign of mechanical elements, or rewiring, as is required with prior art equipment.
Yet another benefit of the present invention is that the same information normally used to control valve operation can be used for diagnostic purposes, both during normal valve operation (e.g., to generate a display or printout indicating the condition which caused the last open or close valve operation to be terminated) and to help evaluate system performance.
It is therefore a primary object of the present invention to provide an improved control system for motor operated valves.
Another object of the present invention is to provide a microprocessor based control and diagnostic system for motor operated valves.
Yet another object of the present invention is to provide a control system for motor operated valves which uses successive valve stem position measurements and valve load measurements to predict the valve stem position and valve stem load at a specified time in the future, and using this capability, takes the inertia of the system into account by turning off the motor when the predicted future value of either the valve stem position and/or the valve load exceeds the corresponding set point.